An automatic transmission shifts itself according to shift patterns or schedules according to vehicle speeds, which are generally known from the speed of an output shaft of an automatic transmission or the speed of traction or driving wheels, and throttle openings which depend upon engine loads. Because an automatic transmission automatically shifts itself to a speed range determined depending upon the vehicle speed and throttle opening but independent from a driver's demand, the automatic transmission occasionally shifts itself to a lower speed range than a speed range to which it is desired to shift in order to prevent the traction wheels from slipping when the vehicle is running, for example, on a slippery road.
To avoid such an undesirable shift of the automatic transmission, a power train has been developed with an improved automatic transmission adapted to shift up when some slippages occurs on the traction wheels or with a control system to control traction force of the traction wheels so as to suppress slippages. Such a power train is known from Japanese Unexamined Patent Publication No.60(1985)-176828.
Traction controlling controls a power train to decrease driving or traction force applied to the traction wheels so as to suppress slippages occurring on the traction wheel. The system of traction controlling is, in more detail, adapted to decreasingly vary the changing rate of opening of a throttle valve per a unit stroke of depression of an accelerator pedal, so as to reduce traction force transmitted to the traction wheels sufficiently less, thereby removing a slippage of the traction wheels.
An automatic transmission controlled by the traction control system, under traction controlling, changes temporarily the running speed of the vehicle or the speed of a traction wheel, which is taken as one of shift control parameters, so that it is, larger than a normal speed depending upon slippages of the traction wheel, and restores the normal speed. Accordingly, if the automatic transmission shifts itself to a shift range determined based on a shift pattern or schedule according to the shift control parameter, the shifted speed range is sometimes undesirable and contrary to a driver's intention or an unnecessary shift is sometimes taken.
Additionally, the opening of a throttle valve, representative of engine load, which is another shift control parameter, does not vary exactly responding to an increase or a decrease of engine load according to a driver's demand, so as also to cause the automatic transmission to shift to an undesirable range, contrary to a driver's intention.